Biosensors are important tools for both research and clinical diagnostics. Since the range of applications for biosensors is broad and the requirements varied, the ideal biosensor technology would combine sensitivity with facile multiplexing and enable the detection and quantification of analytes without the need for a label. It would also be compatible with many different capture agents (e.g. antibodies, aptamers, peptides) and provide real-time detection for the collection of kinetic, as well as equilibrium, binding data. Currently available biosensors do not meet all of these demanding criteria. Here, we propose that a biosensor based on surface plasmon enhanced illumination (SPEI) addresses these issues and thus is expected to have a broad impact in the areas of basic biology, translational research, and clinical diagnostics. The SPEI biosensor that we describe in this proposal is based on measuring changes in the amount of light transmitted through nanoscale (50-200nm) apertures in a metallic surface. We and others have found that the amount of transmitted light depends strongly on the local index of refraction at the sensor surface, which is altered upon the binding of analytes to immobilized capture reagents. While biosensors based on surface plasmon resonance (SPR) also detect changes in the local index of refraction due to biomolecular interactions, the plasmon excitation method is very different for-the SPEI technology. These differences, described in detail in this proposal, allow the active sensing area to be very small, enabling a very high degree of multiplexing, superior resolution, and an increased sensitivity of at least 20 fold over grating coupled SPR. The similarity to conventional SPR, however, allows for the body of applications work done for conventional SPR to be used with SPEI. We propose to design and fabricate a multiplexed SPEI biosensor and demonstrate its performance by detecting and quantifying multiple cytokines in complex media. The specific issues that will be addressed include developing appropriate surface chemistry to reduce nonspecific binding, developing methods that enable the accurate and simultaneous quantification of multiple analytes, and developing methods to optimize the specificity of detection (reduce the incidence of false positives). [unreadable] [unreadable] [unreadable]